Wow, that is fantastic, cant wait to try it out! Is the official name for the pakset "PAK256", or are you going to follow the syntax for the two (okey.. one and a half) existing paksets: Pak256.Japan-Ex?

I believe that there are no "laws" as such what the name should be, but Rollmaterial has a point in that the name could help avoid a bunch of confusion from players who maybe dont know so much about Simutrans (like the difference between Standard and Extended), so I would also suggest to have the -ex in the name.

You write that this pakset is themed toward Japan, however if you look at the "Standard" paksets, you have Pak64 and Pak128, which both are kind of official paksets, and neither really have a country specific location (as far as I know). Most other paksets have in their name specified a country or another thing that makes them special, for instance pak192.Comic, or pak128.Britain etc.I think you could call this pakset pak256.Japan-ex, or something else that makes the pakset name more unique than just "Pak256-ex" (even though, the pakset is most surely already unique in many ways, being the only 256-pixel based pakset, and then for Extended, just to name a few! )

The demo map city names use glyphs that are not part of the English locale font set as such appear as boxes.

The pakset itself is very incomplete. There literally seems to be 3 road vehicles, a few trains and such. There does not even seem to be any railroad electrification despite there being constraints for it (maybe I am looking in wrong place).

Will buildings and vehicles from other countries be included in the pak, or only Japan buildings and vehicles are allowed in this pak?

Just name the pak 'pak.256.JP-ex' had your team limit buildings and vehicles to be based in Japan. It's to indicate that your pak is based in Japan, so that some of us who do not want a Japan pak in mind will not waste time and energy downloading the wrong pak.